A brief, twitching muscular contraction which may involve only a single muscle or many muscles (see MUSCLE). It may be too slight to cause movement of the affected limb, or so violent as to throw the victim to the floor. The cause is not known, but in some cases may be a form of EPILEPSY. A single myoclonic jerk in the upper limbs occasionally occurs in minor motor epilepsy (petit mal). Myoclonic jerks, which many people experience on falling asleep, are a normal phenomenon.